Silicone fluids have been used as lubricants and as surfactants, but their use as rubber processing aids has been limited and/or unsatisfactory.
For example, silicone fluid lubricants have been used as external lubricants or release agents, i.e., they have been applied to the surfaces of molds, dies, etc., but normally they do not possess acceptable compatibility with rubber for internal use. Mixing these silicone lubricants directly (i.e., internally) into rubber compounds, even at small concentrations, can cause serious loss of tack and adhesion. A further disadvantage is that the silicone bleeds out of the vulcanizate.
U.S. Pat. No. 2,720,495 to Phreaner discloses that incompatible silicone fluids can be incorporated into rubber compounds by mixing them with inorganic alkaline earth metal silicates, such as calcium silicate, and a silicone polymerization catalyst, such as calcium magnesium or iron oxide. According to Phreaner, the silicone-silicate filler composition imparts improved resistance to oil-induced swelling, improved rupture and abrasion resistance, and improved plasticity in milling, extruding and molding of the rubber, but no data are provided substantiating those claims. Furthermore, Phreaner does not truly render the silicone fluid compatible with rubber. Rather, Phreaner disperses the silicone fluid on large amounts of silicate filler (ratios of silicate filler to silicone of from 4:1 to 100:1). If the silicone fluid is not sufficiently absorbed by the large amount of silicate, the silicone fluid can separate from the silicate in the rubber compound and cause characteristic silicone defects such as lower adhesion, poorer physical properties, and bleeding of the silicone fluid from the vulcanizate.
Peptizing agents have long been added to rubber compounds. Because crude rubber has a very high viscosity, it cannot be easily and quickly mixed with additives, such as vulcanizing agents, plasticizers, extenders, fillers, pigments, and the like, prior to vulcanization. In order to do so, the crude rubber must first be broken down into a soft, plastic state. This procedure, which is often referred to as mastication, involves mixing rubber with a peptizer (plasticizer) while subjecting the rubber to heat. The peptized rubber is then cooled and then mixed with other additives.
The use of processing aids to facilitate the mixing of rubber and plastic compounds is generally known in the art. For instance, U.S. Pat. No. 3,787,341, Reissue Pat. No. 29,821 to Aron, teaches the use of a certain homogeneous mixture which, along with the other additives, is added directly to crude rubber in the mixing step and which eliminates the need for a separate mastication step. The homogeneous mixture comprises an alkali metal salt of an aromatic sulfonic acid (e.g., potassium dodecylbenzene sulfonate) and a long chain fatty acid (e.g., stearic acid), and is added in an amount of from about 0.5 to about 2 percent by weight of the composition. Preferably, the homogeneous mixture also includes such additional materials as aromatic esters, compounds with alcoholic and glycolic hydroxyl groups, potassium or sodium soaps of fats or fatty acids, other soaps of metals such as zinc, magnesium, calcium or barium, and a paraffin. U.S. Pat. No. 3,882,062 to Aron teaches that thioethers catalyze the action of the homogeneous mixtures.
U.S. Pat. No. 4,534,799 to Aguirre discloses a processing aid which enhances the wetting characteristics of natural and synthetic rubbers comprising a mixture of a partially neutralized, fatty-acid modified alkylbenzene sulfonic acid, a fatty acid amide, a normally-solid polyethylene glycol, a normally-solid low density wax, and a fluorinated surfactant. In addition, the processing aid can include an inert filler and/or carrier.
To date, however, no one has provided a satisfactory rubber processing aid comprising a mixture of a silicone-fluid lubricant and a chemical peptizing agent. Applicant has discovered that such mixtures, when added to rubber, unexpectedly provide unexpected compatibility of silicone lubricants with rubber compounds, and are very effective as rubber processing aids.
For example, basic processing aids of this invention, which comprise a silicone fluid lubricant and a chemical peptizing agent, when added to a rubber compound, improve release, processability, adhesion and tack, but do not adversely affect such other physical properties of the rubber as tensile strength, elongation, modulus, etc. These processing aids can so facilitate the release of rubber compounds that external lubricants need not be applied to the molds in order to release molded parts.
Processing aids comprising a silicone-fluid surfactant in addition to the lubricant and peptizing agent have the additional ability to decrease vulcanization cure time, and thus increase the production rate of vulcanized rubber.
It is one object of this invention to provide a silicone-fluid based processing aid which is truly compatible with rubber compounds, and which does not require silicate fillers.
It is a further object of the present invention to provide a processing aid and a method for incorporating a silicone fluid lubricant into a rubber without adversely affecting the physical properties of the rubber.
It is a further object of the invention to provide a rubber compatible, silicone-based processing aid and a method for imparting excellent release and processing properties to a rubber compound without adversely affecting such properties as adhesion, tack, modulus and tensile strength.
It is yet a further object of the invention to provide a rubber compatible, silicone-based processing aid and a method for increasing the compatibility and dispersion of lubricants in a rubber compound, thereby providing a higher degree of wetting.
It is another object of the present invention to provide a novel rubber compatible, silicone-based processing aid and a method for facilitating the breakdown of a rubber and thereby eliminate the need for a separate mastication step plus a lubrication step.
It is yet another object of the present invention to provide a rubber compatible, silicone-based processing aid and a method for reducing the viscosity of a rubber compound, thereby reducing the mixing time required for homogenizing the rubber compound and providing better dispersion of ingredients in such compounds than have been achieved by use of known processing aids and methods.
It is yet another object of the present invention to impart to cured rubbers an improved resistance to ozone and oxygen degradation and improved physical properties, such as tensile, modulus, elongation, compression set and flexing.
Other objects of the present invention include the provision of improved flow, mixing and extrusion characteristics of rubber and plastic compounds, along with the possibility of uninterrupted working of the mixed compositions, and lower, safer dump temperatures, without adversely affecting the physical properties of the rubbers.
It is a further object of this invention to provide a rubber-compatible, silicone-based processing aid which causes a reduction in vulcanization cure time, and thus permits increased production rates of vulcanized products.
A still further object of this invention is provision of a rubber-compatible, silicone-based processing aid which improves the molding characteristics of the rubber compound and the physical properties of the vulcanized rubber product.
Still another object of this invention is a vulcanized rubber product having improved properties produced through use of the processing aid of this invention.
Another object of this invention is an improved method for producing vulcanized rubber products by mixing the silicone-based processing aid of this invention with a rubber compound, and thereafter molding and vulcanizing the rubber compound.